In vehicles known from prior practice, a drive torque produced by a power source or drive engine is transmitted by a transmission device to the drive wheels of a driven vehicle axle, as necessary. Where vehicles, such as all-wheel passenger cars or all-wheel-drive goods vehicles, are made with more than one driven axle, the power of the engine in the drive train of such a vehicle has to be distributed among the driven vehicle axles.
For this power distribution so-termed differentials are used, which are located in the drive train of a vehicle downstream from a main gearbox provided so that various gear ratios can be engaged. For the longitudinal distribution of the engine's drive power to several driven axles of a vehicle, so-termed longitudinal differentials are used. In addition, so-termed transverse differentials or equalization transmissions are used for the transverse distribution of the drive power between two drive wheels of a vehicle axle.
With the help of such distributor transmissions, a drive torque can be distributed between several driven axles in any desired proportions without producing stresses in a drive train. Moreover, the use of differentials enables the drive wheels of a driven vehicle axle to be driven with different rotation speeds independently of one another in accordance with the different path lengths of the respective left and right driving tracks, whereby the drive torque can be distributed to both drive wheels symmetrically and thus without any yaw torque.
However, these two advantages are offset by the drawback that because of the equalizing action of a differential, the propulsive forces that can be transferred to the road by two drive wheels of a vehicle axle or from two or more driving axles is determined in each case by the lower or lowest transferable drive torque of the two drive wheels or driving axles. This means that when, for example, a drive wheel resting on smooth ice skids, no torque higher than that of the skidding drive wheel can be supplied to the other drive wheel, even when the latter is on ground that it could grip. In such a driving situation, the vehicle might disadvantageously not be able to start off because of the equalizing action of a differential, which allows a difference of speed between two drive output shafts of the differential.
Accordingly in practice, it has become customary to prevent equalization movement of a differential by suitable means in the event of critical driving situations. This is done, for example, by a differential lock, known as such, which can be actuated manually or automatically by mechanical, magnetic, pneumatic or hydraulic means and which fully prevents any equalization movement by blocking the differential.
Furthermore, automatically lacking differentials, also known as equalizing transmissions with limited slip or locking differentials, are used. Such equalizing transmissions make it possible to transfer a torque to a drive wheel of a driven vehicle axle or to a driven vehicle axle even when the other drive wheel, or if there are several driven axles the other driven axles, are skidding because of poor grip on the ground. At the same time, however, the advantage of the above-mentioned yaw-torque-free force transmission is lost and the free adaptation of the wheel rotation speeds to the path lengths of the two driving tracks of the two wheels of a driving axle is also disadvantageously prevented.
WO 02/09966 A1 discloses a transmission for a four-wheel-drive vehicle, in which an input shaft is connected to a planetary gearset. Here, the planetary gearset is made as a three-shaft planetary gearset, such that an annular gear wheel is in active connection with the input shaft, a solar gear wheel with a first drive output shaft, and the planetary carrier with a planetary gear system and with another drive output shaft of the transmission. The planetary gear system comprises three solar gear wheels and three planetary gears each of which meshes with one of the solar gear wheels, which are made integrally with one another and have a common planetary carrier. The planetary carrier of the planetary gear system and one of its solar gear wheels are each in active connection with a brake. These brakes are connected to a force supply and being operated independently of one another and controlled by an electronic control device. To the electronic control device are connected a plurality of sensors, whose signals are received by the electronic control device and converted into corresponding control signals for the two clutches. Depending on the control of the two clutches, the initial speed and the torque transmitted to the front axle and the drive output speed of the planetary gear system and the torque transmitted to the rear axle are adjusted.
However, this all-wheel distributor system, known from the prior art, has the disadvantage that variable torque distribution can only be effected to a limited extent and that its design is elaborate. Owing to its elaborate design, the all-wheel distributor system has large overall dimensions so the all-wheel distributor system takes up more structural space and has a high inherent weight.
Accordingly, the purpose of the present invention is to provide a transmission of simple design that can be made inexpensively and a drive train of a vehicle by way of which a degree of distribution of a drive torque can be varied, as necessary, between at least two driven vehicle axles or between two drive wheels of a driven vehicle axle in such a manner that driving operation of a vehicle is ensured even in critical driving situations.